The invention relates to a liquid transfer device, a control method for maintaining an accurate mixing ratio of liquid mixture by correcting a switching timing of switching valves thereof and a liquid chromatograph with the liquid transfer device. More specifically, it relates to the liquid transfer device including a plurality of switch valves for switching liquids at a predetermined ratio and having a low-pressure gradient function for transferring a mobile phase through sequential suctions of a mixture of liquids by switching the switch valves; a control method for maintaining an accurate mixing ratio of liquid mixture by correcting a switching timing of the switch valves of the liquid transfer device; and a liquid chromatograph including such a liquid transfer device.
A structure of a conventional liquid chromatograph is shown in FIG. 4.
The liquid chromatograph includes a plunger reciprocation type pump having a low-pressure gradient function as a liquid transfer device. The pump is basically formed of a plunger 1, a pump chamber 3, check valves 5, 7 provided on inlet and outlet sides of the pump chamber 3. Further, the pump includes a motor 9, such as a pulse motor, for driving the pump. A cam 11 is driven by the motor 9 thus causing reciprocation movements of the plunger 1.
Liquids to be mixed are liquid A and liquid B, and passages for transferring the respective liquids to the pump chamber 3 are joined together at a merging point 13 on the side of the check valve 5 connected to the pump chamber 3. Switch valves VA and VB are disposed on the respective passages between the merging point 13 and the liquid A and between the merging point 13 and the liquid B.
The switch valves VA and VB are opened or closed by a control portion 15 in synchronization with the suction cycle. More specifically, the control portion 15 obtains a position of the plunger 1 based on a detection signal of a position sensor 17 for detecting a rotation quantity of the motor 9, and controls the switching timing of the switch valves VA and VB in each suction cycle (i.e. during movement of the plunger 1 from a top dead point to a bottom dead point) based on the position of the plunger 1.
The discharge side of the pump chamber 3 is connected to a mixer 19 for mixing the liquids through the check valve 7. A passage from the mixer 19 is connected to one end of a column 23 for separating a sample through an injector 21 for injecting the sample. The other end of the column 23 is connected to an ultra violet detector (hereinafter referred to as UV detector) 25 for detecting the seperated sample.
Hereunder, the control of the switching timing of the switch valves VA and VB will be explained. When the suction cycle starts, the switch valve VA is opened and the switch valve VB is closed. When the plunger 1 reaches a position X, the switch valve VA is closed and the switch valve VB is opened. The position X is determined according to the mobile phase composition, i.e. a mixing ratio of liquid A and liquid B. The mixing ratio is determined beforehand and stored in the control portion 15.
When the opening or closing of the switch valves VA and VB is controlled as described above, liquid A is sucked into the pump chamber 3 during a period from start of the suction to the position X, and liquid B is sucked into the pump chamber 3 after the plunger 1 reaches the position X. Through movement of the plunger 1 from a bottom dead point to a top dead point, the liquid A and liquid B are mixed in the mixer 19 and transferred with a predetermined mixing ratio.
In the plunger reciprocation type pump of a low-pressure gradient system using switch valves, there has been a problem such that in case there is a displacement among the axes of a disk to which the position sensor 17 is disposed, motor 9 and cam 11, the suction start point, i.e. top dead point, is also displaced, so that it is impossible to obtain an accurate mixing ratio. Also, there has been a problem such that in case there is a machining error of the cam 11, a response delay, or a discrepancy in response speeds at the timing of opening or closing of the respective switch valves, an error is made between the predetermined mixing ratio and the mixing ratio of the liquids actually transferred. Further, there has been a problem such that the mixing ratio error as described above is different in every device.
In view of the above problems, the present invention has been made and an object of the invention is to provide a correcting method for controlling an error between a mixing ratio of an actually transferred mobile phase and a predetermined mixing ratio of the mobile phase.
Another object of the invention is to provide a liquid transfer device having a function as described above.
A further object of the invention is to provide a liquid chromatograph having the function as described above.
Further objects and advantages of the invention will be apparent from the following description of the invention.